Designing sensors that can measure and survive shock loads in extremely harsh real-world environments, while still providing a high signal to noise (S/N) ratio and significant bandwidth, can be difficult. Further, a small sensor form factor (e.g. a microelectromechanical systems (MEMS) sensor) can be necessary to prevent the sensor deleteriously affecting the structure under load, while facilitating arrayed and embedded applications. Possible applications for a very small sensor that can survive high shock loads and provide a high S/N ratio with a high bandwidth can include crash-test dummy instrumentation, impact forensics, embedded sensors in smart surfaces for lifetime structural health monitoring, etc. Such sensors can indicate when a structural element (e.g., an aircraft wing, a bridge pylon, etc.) has experienced an out-of-bounds load due to an extreme event (e.g., an explosion, an earthquake, etc.). A small sensor can also find application measuring a system response to impact loads, as well as for validating modeling and simulation tools.
There are three main types of normal load (pressure) sensors: capacitive, piezoelectric, and piezoresistive. Each type has performance and manufacturing advantages and shortcomings associated therewith. Capacitive sensors typically have a very small signal (e.g., change in capacitance) relative to the overall sensor capacitance associated with the complete circuit. Accordingly, a parasitic capacitance associated with signal lines on a chip separated from a chip ground by a thin oxide layer can be difficult to avoid as the ground is intimately connected to the sensing element. Such factors tend to reduce the signal to noise ratio for a capacitive sensor. Further, tight tolerances on the small sensing capacitor gap can also be problematic as a very small gap is desirable to achieve higher sensitivity, however a very small gap can limit a range of gap deformation and further lead to an increase in sensor response signal variability as a function of dimensional variability of the gap.
Piezoelectric sensing elements are typically quartz crystals which require poling and assembly, including bonding of the crystal into an electronic package. Recent advances in the fabrication of piezoelectric thin films are facilitating monolithic fabrication of piezoelectric sensors with a reduction in the reliance on adhesives at the critical sensing element level. An advantage of piezoelectric sensors is that they can generate an output voltage with no input voltage required, although an input voltage is required for any in-package signal amplification or processing electronics. However, piezoelectric sensors lack a robustness required to survive dynamic shock loads of a high magnitude.
Typically a DC signal (e.g., a low frequency response) is not sensed in a piezoelectric sensor, while it can be for a piezoresistive sensor. Additionally, monolithic fabrication techniques for piezoresistive silicon (Si) sensing elements are well developed. Si has a large piezoresistive gage factor, can be mass-fabricated, has a high elastic modulus and possesses a high ultimate strength in uniaxial compression. However, Si is a brittle material with tensile and flexural strengths being much lower in comparison with its compressive strength performance. Such material properties can render measurement of highly dynamic loads problematic with a Si sensor material. An application which can be particularly problematic is where dynamic loading can induce tensile stress waves into structural components which can lead to the generation of microcracking in the Si sensor material.